4 erasers cost $5.20. Which equation would help determine the cost of 13 erasers?
Solution: There are several equations that could help determine the cost, each with a slightly different approach. We know the cost of 4 erasers. We want to know the cost of 13 erasers. We can write the numbers of erasers as a proportion: $\dfrac{4}{13}$ We know 4 erasers costs $5.20. We can let $x$ represent the unknown cost of 13 erasers. The proportion of these costs can be expressed as: $\dfrac{\$5.20}{x}$ The cost changes along with the number of erasers purchased, and so the two proportions are equivalent. $\dfrac{4}{13} = \dfrac{\$5.20}{x}$